1. Field of the Invention
The present invention relates to copying machines, notably of the type permitting obtaining a concave surface on an article such as a lens blank, and has specific reference to a grinding machine operating according to the copying principle for making concave surfaces of ophtalmic lenses.
2. Description of the Prior Art
Grinding machines are already known which operate according to the copying principle and permit grinding the concave surfaces of ophthalmic lenses, especially the aspheric convex surfaces of ophthalmic lenses of the type wherein the surface curvature varies continuously from one edge to the opposite edge of the lens in order to impart thereto a focal power varying as the axis of vision scans the lens, for instance from the upper edge to the lower edge thereof. For instance such lenses, usually referred to as variable focal power lenses, are known in the trade under the name of "Varilux". Known grinding machines designed for obtaining these non-spherical surfaces comprise as a rule a working table, an interchangeable templet having a convex surface corresponding to the surface to be made on a lens blank, a pair of supports for the templet and for the lens blank respectively, which are rotatably mounted side by side on the table and driven for synchronous rotation about two parallel axes merging into the templet axis and the optical axis of the blank on which the desired convex surface is to be made, respectively, and a movable copying head carrying a feeler for making a pin-point contact with the templet, and a rotatably driven grinding wheel. In grinding machines of this type both the grinding wheel and the feeler have a cylindrical shape and have the same radius. The copying head is pivotally mounted by means of a generally horizontal shaft, or pair of trunnions, on a carriage movable in a direction perpendicular to the plane defined by the generally vertical axes of rotation of said pair of supports. With this arrangement, the feeler and the grinding wheel bear by gravity on the templet and the lens blank, respectively, and since the latter are rotatably driven together with their respective supports, the feeler and grinding wheel describe each a spiral path respectively on the templet and on the lens blank when the carriage is moved, thus reproducing the entire convex surface of said templet.
Although known grinding machines of the above-described type are very satisfactory for making convex aspheric surfaces, they are not suited for obtaining concave surfaces. This is due to two reasons, the first one being that it is not possible to obtain a pin-point contact between a cylindrical feeler and a templet having a concave surface. A first modification is therefore necessary, namely, the use of a feeler and of a grinding wheel having both a convex surface of revolution, for example a sperical or toroidal grinding wheel. Moreover, the feeler and grinding wheel should have a radius of curvature smaller than the smallest radius of curvature of the concave surface of the templet. However, even if these modifications were adopted and if the above-mentioned requirements were met, hitherto known grinding machines thus modified would nevertheless still not operate satisfactorily for grinding concave surfaces for various reasons which will now be explained with reference to FIG. 1 of the attached drawings, illustrating diagrammatically by way of example a known grinding machine equipped with a sperical grinding wheel and used for grinding a concave surface.
In this FIG. 1 the reference numeral 1 designates a lens blank on the top surface of which a concave surface S is to be obtained. A support 2 for the blank 1 is rotatably driven about a vertical axis, and a grinding wheel, for example a spherical wheel, is mounted to one end of a rotatably driven spindle 4 mounted for rotation in a copying head shown diagrammatically in the form of an arm 5. This arm 5 is pivotally mounted on a carriage 6 adapted to travel horizontally, i.e. at right angles to the axis of rotation of support 2. To facilitate the understanding it will be assumed firstly in the following disclosure that the concave surface S to be obtained is a portion of a sphere centered to the optical axis of blank 1. It will be seen that when the grinding wheel 3 is substantially located centrally of blank 1, it bears there against in a direction substantially perpendicular to the concave surface of the blank. On the other hand, when the grinding wheel 3 is on the marginal portion of blank 1, after the carriage 6 has been moved from the position shown in thick lines to the positions shown in dash and dot lines in FIG. 1, the wheel 3 bears against the blank 1 in a direction which is no longer perpendicular to the concave surface of this blank. As a result, there is a risk of chipping, spalling or flaking the marginal portion of the glass blank 1 as a consequence of the force applied by the grinding wheel on the blank.
The smaller the radius of curvature of surface S, the higher the risk of spalling the edge of the glass blank.
Moreover, it will be seen that when the grinding wheel 3 is located substantially centrally of the blank 1, it is the small spherical zone 3a of wheel 3 that removes material from the blank 1, whereas when the wheel 3 is grinding the area near or at the edge of the blank 1, the glass is ground by the small spherical zone 3b of the wheel. In other words, when the carriage 6 is moved from the position shown in thick lines to the position shown in dash and dot lines of FIG. 1, the working or useful spherical area of wheel 3 is shifted from zone 3a to zone 3b. Therefore, the wheel 3 is worn on a relatively wide spherical surface area bounded by small zones 3a and 3b. Since the direction in which the wheel 3 is pressed against the blank 1 varies as the wheel 3 moves from the centre to the edges the blank, the degree of wear of the grinding wheel 3 is not the same or uniform throughout the width of the working area defined by said small zones 3a and 3b. Consequently, it is not possible to compensate wear as in the case of a working area (as seen on the grinding wheel itself) remaining constantly the same. With such a known grinding machine, it is clear that concave surfaces cannot be obtained with a sufficient degree of fidelity, unless the grinding wheel is replaced very frequently.
It is obvious that the above-described known grinding machine has the same drawbacks, possibly amplified, when the concave surface S of blank 1 is a toroidal or aspheric surface, for example the aspherical surface of a Varilux type ophthalmic lens. Moreover, in case the surface S were a toroidal or aspheric surface, the working area of the grinding wheel, when the latter is in any intermediate position between the two positions illustrated in FIG. 1, is not the same for all the possible angular positions of support 2 and blank 1, because the radius of curvature of the concave surface S has not the same value for all the sections of blank 1 taken through planes containing the optical axis. Assuming that the carriage 6 remains stationary, the distance between the centre 0 of surface S and the point of contact between the grinding wheel 3 and the surface S will thus vary during one revolution of support 2. Consequently, when the support 2 rotates and the carriage 6 is moved slowly, the point of contact between the grinding wheel 3 and the surface S describes on the latter a spiral having a variable pitch. Therefore, the surface S is not ground regularly. Moreover, when the surface S is of toroidal configuration with its two main radii having different values, the spiral thus described is elongated considerably in one direction and somwhat flattened in the direction perpendicular to said one direction, thus requiring the use of a templet which, when seen in plane view, has an elongated configuration.
For all the reasons set forth hereinabove, it is not desirable to use the known grinding machines operating according to the copying principle, even if a spherical or toroidal grinding wheel is adapted thereto, for obtaining concave surfaces.